Spectroscopy dispersed emission spectrum

At this point it is relevant to note the terminology employed in this chapter the expression laser-induced fluorescence (LIF) is used as a general term describing any fluorescence that is excited using a laser. A fluorescence excitation spectrum shows fluorescence emission yield as a function of excitation wavelength that is, it is similar to an absorption spectrum when that absorption results in radiative emission. It is noted that some authors reserve LIF as a synonym for fluorescence excitation spectroscopy. Dispersed fluorescence refers to dispersion of the emitted fluorescence light into its component wavelengths, that is, production of an emission spectrum. 

Figure 7 Schematic illustration of two high-resolution laser techniques. On the left is a technique that relies on excitation (or absorption) spectroscopy, spectral hole-burning, which is therefore instrumentally limited only by laser line width. On the right, fluorescence line narrowing is indicated, a technique that involves recording an emission spectrum which is therefore limited both by laser line width and dispersion resolution.

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

Although Dispersed Fluorescence (DF) spectroscopy is probably better classified as a form of double resonance spectroscopy, DF is discussed here because it is a form of emission spectroscopy where all of the emission originates from a single, laser-populated, upper electronic-vibrational-rotational level, (e, v, J ). A DF spectrum typically contains two [R J" = J — 1), P(J" = J + 1)] or three [i ( J — 1), Q(J ), P J +1)] rotational transitions per electronic-vibrational e",v" level. Often there is a progression of vibrational bands, [ v, v" = n), (v, v" = n + 1),. .. (v, v" = n + to)] where v" = n is the lowest vibrational level (band farthest to the blue) and v" = n + m is the highest vibrational level observable (limited either by the detector response or Franck-Condon factors) in the DF spectrum (see Fig. 1.8 and Fig. 1.15). 

The chemical composition of the natural beryl sample used in this study was analyzed by X-ray wavelength dispersive spectroscopy for major atomic contents, inductivity coupled plasma-atomic emission spectroscopy for Be content, and atomic absorption sp>ectroscopy for Li and Rb contents (Table 1). The type I/II H2O contents were determined from intensities of IR bands due to the asymmetric stretching of type I and the symmetric stretching of type II in a polarized IR spectrum at RT (See the spectrum in the next section), using their molar absorption coefficients of 206 L moH cm-i and 256 L moH cm-i. 

In sections 4.2 and 4.4 emission spectroscopy provided more information than could be obtained from an excitation spectrum. Even so, the weakness of such emission makes it difficult to obtain high resolution in its dispersed spectrum, and data are only obtained about levels lying below that excited. Two-colour double resonance can remove both these limitations in favourable cases. We may envisage three basic schemes for optical-optical double resonance using two lasers. 

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